Journal of Spectroscopy

Volume 2013 (2013), Article ID 702310, 7 pages

http://dx.doi.org/10.1155/2013/702310

Research Article

Synthesis, Characterization, and Photochemical Properties of a New Square Mn(I)-Ru(II) Complex Using Pyrazine as Bridge Ligand

Inara de Aguiar,¹ Simone D. Inglez,² Antonio Claudio Tedesco,³ and Rose Maria Carlos¹

¹Departamento de Química, Universidade Federal de São Carlos, CP 676, 13565-905 São Carlos, SP, Brazil

²Faculdade de Ciências Exatas e Tecnologia, Universidade Federal da Grande Dourados, CP 533, 79804-970 Dourados, MS, Brazil

³Departamento de Ciências Farmacêuticas, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, 14040-901 Ribeirão Preto, SP, Brazil

Received 1 July 2012; Revised 19 September 2012; Accepted 8 October 2012

Academic Editor: R. P. S. Chakradhar

Copyright © 2013 Inara de Aguiar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The photochemical properties of the complexes cis,fac- (I), cis-[Ru(phen)2(pz)₂]²⁺ (II), and fac-Mn(CO)₃(pz)₂Br (III) where phen is phenanthroline and pz is pyrazine in acetonitrile solution are reported. The three complexes were characterized using ¹H NMR, UV-vis and FTIR spectroscopy and electrochemical (cyclic voltammetry and spectroelectrochemical) techniques. The complexes show intense absorption in the visible region assigned to the population of MLCT excited states. The absorption spectrum of I is the sum of the spectra of the mononuclear species II and III, and the two oxidation potentials at +1.10 and +1.56 V versus Ag/AGCl observed in I are ascribed to the different coordination environments of metal centers. The photolysis in the acetonitrile solution resulted in the pz dissociation to give the monoacetonitrile complexes for I, II, and III, respectively.

1. Introduction

The investigation of spectroscopic, electrochemical, and photochemical properties of manganese compounds has attracted much attention due to the potential application of these compounds in the development of the supramolecular system which may work photochemically for clean energy sources in renewable solar fuels [1, 2]. Most of current research is being focused on the covalent coupling of a photoactive Ru(II) polypyridinic complex to a high-valence mono- and/or multinuclear μ-oxo bridged manganese complexes. Herein, we wish to report a novel, square pyrazine donor-acceptor complex, composed of a triscarbonyl manganese complex linked to two ruthenium(II) phenanthroline complexes via two pyrazine bridge ligands. Our approach, introducing three good π acceptor (CO) and one good π donor (Br⁻) ligand, was chosen because of the abilities of these ligands to accept and donate electronic density to metal centers and in this way to stabilize the high oxidation states that the manganese center may acquire during the photoinduced electron transfer reaction. For this reason, in this work the photochemical stability of the complexes was studied.

2. Experimental

2.1. General

All synthesis and electrochemical and spectroscopic experiments were carried out under purified N₂ atmosphere, using Schlenk techniques. RuCl₃·xH₂O, 1,10′-phenanthroline (phen), pyrazine (pz), and lithium chloride were from Aldrich; tetrabutylammonium hexafluorophosphate (TBAPF₆) and bromide pentacarbonyl manganese from Strem. HPLC grade acetonitrile and dichloromethane were distilled prior to use. The solutions were carefully handled in the dark before the experiments were performed. The complexes cis-[Ru(phen)₂Cl₂]·2H₂O [3], cis-[Ru(phen)₂CO₃]·2H₂O [4], cis-[Ru(phen)₂(H₂O)₂](PF₆)₂ [5], and [Mn(CO)₃(pz)₂Br] [6] were prepared by the literature routes. FTIR spectra were measured in CaF₂ windows in CH₂Cl₂ solution on a Bomem-Michelson 102 spectrometer in the 4000–1000 cm⁻¹ region. UV-visible spectra were recorded on an HP-8453 A (Diode array) spectrophotometer. NMR spectra were recorded using a Bruker DRX400 spectrometer. All chemical shifts (δ) are given in ppm units with reference to the hydrogen signal of the methyl group of tetramethylsilane (TMS) as internal standard. Monochromatic irradiations at 350 nm and 420 nm were generated using an RMR-600 model Rayonet Photochemical reactor using RMR-3500 and RMR-4200 lamps, respectively. The continuous photolysis experiments were followed by UV-vis. Time-resolved optical spectra were obtained using a laser flash-photolysis apparatus containing a Continuum Q-switched Nd:YAG laser (Continuum, Santa Clara, CA) with excitation provided by the third harmonic at λ 355 nm. Cyclic voltammetry was performed using an μ Autolab Type III potentiostat. Voltammograms were obtained in CH₃CN (1 mM TBPF₆) at 22°C in a light-protected voltammetric cell with a platinum cylinder disc for both the working and the auxiliary electrodes. A silver wire coated with silver chloride was used as reference electrode, connected to the bulk of the solution by a Luggin capillary filled with the same solvent and electrolyte. Solutions were deoxygenated with a stream of N₂ and maintained under a positive pressure of N₂ during the measurements. The concentration of the complexes was kept always at 1 mM.

2.2. Synthesis

2.2.1. cis,fac-[Ru(phen)₂(pz)₂-Mn(CO)₃Br]₂(PF₆)₄(I)

Mn(CO)₅Br (74 mg, 0.27 mmol) was dissolved in degassed acetone (50 mL), and the complex cis-[Ru(phen)₂(pz)₂] (250 mg, 0.27 mmol) was added. It was stirred under dark for 12 hours at room temperature. The orange precipitate formed was filtrated and dried under vacuum. Yield = 80%.

2.2.2. cis-[Ru(phen)₂(pz)₂](PF₆)₂(II)

[Ru(phen)₂Cl₂]·2H₂O (200 mg, 0.35 mmol) and pyrazine (282 mg, 3.52 mmol) were refluxed in a 1 : 1 EtOH/H₂O solution for 6 hours. Upon cooling to room temperature, a saturated solution of ammonium hexafluorophosphate was added to the solution to precipitate an orange product, which was isolated by filtration. The orange solid was recrystallized using acetone/ether. Yield: 70%.

2.2.3. fac-[Mn(CO)₃(pz)₂Br](III)

Mn(CO)₅Br (200 mg, 0.73 mmol) was dissolved in degassed CH₂Cl₂ (50 mL) and pyrazine (117 mg, 1.45 mmol). It was stirred under dark for 12 hours at room temperature. The yellow precipitate formed was filtrate and dried under vacuum. Yield: 80%.

3. Results and Discussion

The square complex was synthesized in a manner similar to a previously published procedure [1], starting from cis-[Ru(phen)₂(pz)₂]²⁺ and considering the complex Mn(CO)₅Br as a ligand (Scheme 1).

Scheme 1: Synthesis of square complex I.

The ¹H NMR spectral data for the ligands and complexes in CD₃CN are listed in Table 1 using the numbering scheme as represented for complex II as follow. The signals of the complex I were assigned by comparison and analysis of the precursor Ru(phen)₂Cl₂ and the free pyrazine. In the complex I, the phenanthroline protons appeared as eight signals instead of four signals to the free ligand, as expected for the cis isomer. The two pyrazine α and β protons were observed at 8.49 (4H, d) and 8.45 (4H, d). On coordination of Ru(II) to Mn(I) (Figure 2, Table 1), the linewidth of all signals were broadened due to the presence of the bromide ion coordinated at the manganese center. The absence of new signals in the whole spectrum, as expected to a triangle complex, suggests the formation of the square complex, which was further confirmed by signals integration [7]. In the square complex, all the assignments were done using the same numbering figure as complex II, Figure 1.

Table 1: 1H NMR spectral data of complexes I and II in CD₃CN.

(ppm)
Complex II
Complex I

Proton type

H₁  
9.35 (2H, d)
9.35 (4H)

8.25 (4H, m)
8.26 (8H)

8.84 (2H, d)
8.84 (4H)

8.25 (4H, m)
8.26 (8H)

8.14 (2H, d)
8.15 (4H)

8.50 (2H, d)
8.50 (4H)

7.57 (2H, m)
7.58 (4H)

7.98 (2H, dd)
7.98 (4H)

8.49 (4H, d)
8.50 (8H)

8.45 (4H, d)
8.50 (8H)

(a)

(b)

Figure 1: ¹H NMR spectra in CD₃CN of the aromatic region to the complex II (a) and complex I (b).

Figure 2: Electronic spectra in CH₃CN solution of (I) cis,fac-[Ru(phen)₂(pz)₂-Mn(CO)₃Br]₂, (II) cis-[Ru(phen)₂(pz)₂], (III) fac-[Mn(CO)₃(pz)₂Br].

3.1. Absorption Properties

The absorption spectra of complexes I, II, and III in CH₃CN solution are shown in Figure 2. The absorption maximum of 380 nm ( mol⁻¹ L cm⁻¹) for III appears as a shoulder on the strong π-π* absorption bands of the ligands, while the complex II exhibits the intense and broad MLCT absorption ( nm;  mol⁻¹ L cm⁻¹) typical of polypyridine complexes [8]. The absorption spectrum of I ( nm;  mol⁻¹ L cm⁻¹) is consistent with the superposition of bands characteristics of the corresponding mononuclear complexes. The data for complex I show that the energy of the charge transfer band (394 e 442 nm) remains unchanged upon the introduction of Mn ion into the Ru complex, whereas it is slightly shifted to shorter wavelength compared to the tris-(phenanthroline) complexes (422 and 446 nm) [9], which is characteristic of ligands serving as better π-acceptors ligands than phen.

The FTIR spectrum of III, shown in Figure 3, exhibitsthree intense v(CO) absorptions at 2041, 1953, and 1932 cm⁻¹ consistent with the facial arrangement of the three COs in the coordination sphere. For complex I, the v(CO) stretching frequency appears as weak and broad bands around 2032 and 1940 cm⁻¹, and the two lower energy bands are overlapped suggesting the attachment of Mn(CO)₃ unit into the complex.

Figure 3: FTIR spectra of complexes III (black line) and I (red line) in CH₂Cl₂ at room temperature, with CaF₂ window.

3.2. Electrochemistry

The voltammetric data are summarized in Table 2. Figure 4 shows a cyclic voltammogram (scan rate 100 mVs⁻¹) for a 1 mM solution of the complexes I, II, and III over the range 0–1.8 V (versus AG/AGCl) in acetonitrile (TBPF₆ 1 mM).

Table 2: Electrochemistry properties of complexes I, II, and III in CH₃CN.

/,
νCO cm⁻¹
, nm ()

Complex I
1.10
2032, 1938
394 (11500);

0.79/0.73
442 (10400)

Complex II
1.56/1.49
—
394 (9100)

425 (8000)

Complex III
1.10
2041, 1953, 1932
381 (2900)

0.87/0.89

Figure 4: Cyclic voltammograms of complexes I, II and III (1 mM), TBPF₆ (0.1 M) versus Ag/AgCl, scan rate 100 mV/s, 25°C, in CH₃CN.

Complex II exhibits a redox couple at  V ( V and  V versus Ag/AgCl) of which is more positive than those found to [Ru(phen)₃]²⁺ [10]. The complex III, on the other hand, displays a shoulder at 0.80 V corresponding to oxidation followed by an oxidation peak at 1.10 V attributed to the oxidation of to , which is paired with a nonreversible reductive wave at 0.90 due to reduction.

The Mn(CO)₃Br coordination on the Ru(phen)₂(pz)₂ did not change the redox potential values of the Ru(II) metal center. The cyclic voltam ogram of complex I exhibits one irreversible oxidation at 1.10 V and two redox couple  V ( V and  V versus Ag/AgCl) and  V ( V and  V versus Ag/AgCl).

3.3. Spectroelectrochemistry

Insights into the bonding characteristics of the Ru(II) complexes for complexes I and II were obtained by spectroelectrochemical experiments. For the Ru(II) complexes, a constant potential 1.5 V (determined from cyclic voltammetry) was applied and the extent of oxidation to Ru(III) was monitored by UV-vis spectroscopy (Figure 5). The spectrum shows the disappearance of the broad absorption band at 400 nm. After 30 min of oxidative electrolysis, the spectral changes were completed. The oxidative spectroelectrochemistry at 1.5 V leads to disappearance of the MLCT band indicating that oxidation was a process which is irreversible.

(a)

(b)

Figure 5: Spectroelectrochemical of complex I (1.5 mM in CH₃CN; 0.1 M TBAPF₆) and complex II (1.5 mM in CH₃CN; 0.1 M TBAPF₆) in an OTTLE cell, oxidation  V versus Ag/AgCl.

3.4. Transient Absorption Spectra

Figure 6 shows the excited state absorption spectrum for complex I in CH₃CN solution after excitation with an 8 ns pulse at 355 nm irradiation. There is a bleach of absorption band at 400 nm and new structured absorption with maxima centered at 350 nm and 600 nm consistent with formation of an MLCT (Ru → phen) excited state [11].

Figure 6: Transient (upper) and experimental (low) absorption spectra of complex I in acetonitrile.

3.5. Photochemistry

The complexes are stable in deaerated solutions in the absence of light. When solutions of complex II were subject to continuous photolysis, the resulting optical spectral changes were consistent with the substitution of only one pyrazine molecule by a solvent molecule (1) and (2)  

For example, Figure 7 illustrates the spectral changes seen when an acetonitrile solution of complex I (0.22 mM) was irradiated at  nm, einstein s⁻¹. The spectra show a progressive depletion of the characteristic absorption band at 400 nm concomitant with formation of two broad shoulders at 385 and 422 nm, in accordance with the formation of complex Ru(phen)₂(CH₃CN)₂ [12]. For complexes I and II, exhaustive photolysis leads to the same final stable spectrum assigned to the monosolvated complex.

(a)

(b)

(c)

Figure 7: Photochemistry of complex II (1.5 mM in CH₃CN,  nm), complex III (0.95 mM in CH₃CN,  nm), and complex I (1.5 mM in CH₃CN,  nm).

3.6. Photoinduced Electron Transfer Reactions

Figure 8 shows the UV-vis spectral changes of the thermal reaction of a solution containing complex III and MV²⁺ (methyl viologen) in pure water immediately after 10 s continuous irradiation at 355 nm light. Before irradiation, the absorption spectrum of the mixture shows the characteristic absorption of starting complex ( nm). A broad absorption band with maximum near 605 nm and a peak at 394 nm appeared just after irradiation. These new absorptions match the methyl viologen radical absorption spectrum [13].

Figure 8: Spectral changes accompanying the consume of reduced methyl viologen , from the thermal reaction in H₂O, (black line—spectrum of the mixture: complex III and MV²⁺ before irradiation).

The photoinduced electron transfer reactions for complexes I and II did not occur, since in the UV-vis spectra was not observed any characteristic band even after exhaustive photolysis. These results show that the intermolecular photoinduced electron transfer reaction is activated in water only in certain conditions. The presence of Ru(II) unit inhibits the MV²⁺ reduction.

References

I. De Aguiar, S. D. Inglez, F. C. A. Lima et al., “Photochemical reactions of fac-[Mn(CO)₃(phen)imidazole]⁺: evidence for long-lived radical species intermediates,” Inorganic Chemistry, vol. 47, no. 24, pp. 11519–11526, 2008.

V. C. Nogueira, C. Longo, A. F. Nogueira, M. A. Soto-Oviedo, and M. A. D. Paoli, “Solid-state dye-sensitized solar cell: improved performance and stability using a plasticized polymer electrolyte,” Journal of Photochemistry and Photobiology A, vol. 181, no. 2-3, pp. 226–232, 2006.

B. P. Sullivan, D. J. Salmon, and T. J. Meyer, “Mixed phosphine 2,-bipyridine complexes of ruthenium,” Inorganic Chemistry, vol. 17, no. 12, pp. 3334–3341, 1978.

E. C. Johnson, B. P. Sullivan, D. J. Salmon, S. A. Adeyemi, and T. J. Meyer, “Synthesis and properties of the chloro-bridged dimer [(bpy)₂RuCl]₂²⁺ and its transient 3+ mixed-valence ion,” Inorganic Chemistry, vol. 17, no. 8, pp. 2211–2215, 1978.

P. Bonneson, J. L. Walsh, W. T. Pennington, A. W. Cordes, and B. Durham, “Six-coordinate complexes with 1,10-phenanthroline ligands in the trans configuration. Preparation of trans-bis(1,10-phenanthroline)ruthenium(II) complexes and crystal structure of trans-bis(1,10-phenanthroline)bis(pyridine)ruthenium(II) hexafluorophosphate,” Inorganic Chemistry, vol. 22, no. 12, pp. 1761–1765, 1983.

R. M. Carlos, I. Ap. Carlos, B. S. Lima Neto, and M. G. Neumann, “Spectroscopic and electrochemical properties of [Mn(phen)(CO)₃(imidazole)](SO₃CF₃) complexes,” Inorganica Chimica Acta, vol. 299, no. 2, pp. 231–237, 2000.

L. A. Berben, M. C. Faia, N. R. M. Crawford, and J. R. Long, “Angle-dependent electronic effects in 4,-bipyridine-bridged Ru₃ triangle and Ru₄ square complexes,” Inorganic Chemistry, vol. 45, no. 16, pp. 6378–6386, 2006.

K. Kalyanasundaram, Photochemistry of Polypiridyne and Porphyrin Complexes, Academic Press, London, UK, 1992.

C.-T. Lin, W. Böttcher, M. Chou, C. Creutz, and N. Sutin, “Mechanism of the quenching of the emission of substituted polypyridineruthenium(II) complexes by iron(III), chromium(III), and europium(III) ions,” Journal of the American Chemical Society, vol. 98, no. 21, pp. 6536–6544, 1976.

F. Barigelletti, A. Juris, V. Balzani, P. Belser, and A. Von Zelewsky, “Influence of the ligand structure on the electrochemical and spectroscopic properties of ruthenium(II)-polypyridine complexes,” Inorganic Chemistry, vol. 26, no. 24, pp. 4115–4119, 1987.

C. Creutz, M. Chou, T. L. Netzel, M. Okumura, and N. Sutin, “Lifetimes, spectra, and quenching of the excited states of polypyridine complexes of iron(II), ruthenium(II), and osmium(II),” Journal of the American Chemical Society, vol. 102, no. 4, pp. 1309–1319, 1980.

P. Bonneson, J. L. Walsh, W. T. Pennington, A. W. Cordes, and B. Durham, “Six-coordinate complexes with 1,10-phenanthroline ligands in the trans configuration. Preparation of trans-bis(1,10-phenanthroline)ruthenium(II) complexes and crystal structure of trans-bis(1,10-phenanthroline)bis(pyridine)ruthenium(II) hexafluorophosphate,” Inorganic Chemistry, vol. 22, no. 12, pp. 1761–1765, 1983.

T. Watanabe and K. Honda, “Measurement of the extinction coefficient of the methyl viologen cation radical and the efficiency of its formation by semiconductor photocatalysis,” Journal of Physical Chemistry, vol. 86, no. 14, pp. 2617–2619, 1982.